June 23, 2010: OPAL development has officially ended. (The code repository will remain on SourceForge for some time, but any new projects should avoid using OPAL.) At the time when this project started, it served its purpose: an open source, cross-platform, simple API for physics simulation, mainly as a simple interface to the ODE simulation library. Now there are other more robust alternatives (e.g. the excellent Bullet physics library). For people looking for an abstraction layer around multiple physics libraries, we suggest the Physics Abstraction Layer project.